The size of samples being analyzed by liquid chromatography has been decreasing. The need to segregate and handle smaller sample volumes and to control the position of the sample in the face of higher pressures and leakage has increased. One area where this is especially important is in the sample injector portion of an in-line liquid chromatography instrument. In-line operation of a liquid chromatograph involves automated selection of samples that are sequentially drawn into a needle or capillary and then loaded into a sample loop by pulling the fluid through the needle and any associated tubes and valves into the sample loop. After the sample is in the sample loop, the sample loop is connected to an injection mechanism, such as a pump/detector system, that pushes the sample through a liquid chromatography column where the separation takes place. The sample can be pulled through the system of tubes at a flow rate that is directly related to the vapor pressure of the fluid. If the fluid is drawn through the tubing too quickly, the fluid can vaporize and cause undesirable effects on sample integrity as well as sample positioning within the sample loop. With in-line operation however, the vapor pressure of the samples can vary. Currently the drawing mechanism has to be set to a speed that will handle all the anticipated samples. It would be advantageous to transport fluid at the optimum speed even when the viscosity of the fluid changes within a sequence of fluids.
Liquids that are drawn through a fluid path encounter friction at the walls of the tubes making up the fluid path. This causes some mixing of the liquids as the friction drags at the liquid next to the walls. When a sample bracketed by air gaps, as is known in the industry, passes through a fluid path, some of the liquid ahead of the bracketed sample transfers across the air gap causing the volume of fluid ahead of the sample to change. As the diameter of the fluid path decreases, this change in volume causes an inaccuracy in positioning of the samples.
All of the components of the injector are replaceable and must be cleaned between injections. Because there are many joints and connections, there is an opportunity for leaks to develop. These leaks may be small enough that they evaporate before they can be visually detected. However, the leakage will degrade the accuracy of the injection by causing undesirable sample movement. Early detection of such leaks would allow scheduled replacement of parts and improve the quality of the operation of the instrument.
A further consequence of the replaceable components utilized in the injector is the inaccuracy that accrues due to the tolerance of the components. In general, components such as an aspirating needle and a sample loop have an internal volume manufactured to a tolerance that is not as precise as desired. While the internal volumes are manufactured to fall within a predetermined range, the range is too broad to assure accurate positioning of a sample within the sample loop. As the parts are replaced, the control mechanism cannot use small volumes of sample because it cannot be sure of the volume taken up by the needle and/or where that volume is positioned in the sample loop. The volume of the replacement part is not specified with sufficient precision to allow the sample to be positioned in the sample loop accurately. The tolerance problem especially effects partial loop injections.
The systems can suffer from changes in volume that impede accuracy. Deposits that build up in the tubing change the volume of the system, can create turbulence within the tubing and therefore degrade performance. The variability of the volume of these components makes it hard to minimize the amount of sample being used while providing a precise amount of sample to the column.
Each of these problems and challenges of handling small volumes of sample point to a need for a device that provides more information about the fluid system.